TCP-FCW – transport protocol for real-time transmissions on high-loss networks Sergei Kozlov,

2 Research Agenda RTP/RTCP versus TCP TCP-RTM (TCP Real-Time Mode) TCP-RTM and bursty losses Free congestion window (FCW) concept Short evaluation of TCP-FCW Very briefly – future work

3 Research RTP/RTCP RTP is widely used for RT streaming RTP = UDP + time-stamping –no reliability – what is lost is lost –no in-sequence guarantees – upper layers should take care of it –low end-to-end delay, low jitter RTCP – a protocol for controlling the stream –feedback on the quality of data –delivery to the sender of a participants canonical name –awareness of participants about each other

4 Research TCP (I) Guaranteed delivery –what you send is what you receive (sooner or later…) Sophisticated retransmission mechanism –based on RTT measurements, duplicate acknowledgments, timeouts etc. Congestion-control mechanism –this is how it stays non-aggressive to the others Its designed for reliable data transfers –favors reliability over timeliness –may introduce unwanted delays for RT data

5 Research TCP (II) – retransmissions and sliding window Straight-forward acknowledgments –Ineffective usage of the network Sliding window (snd_wnd) data ack data ack senderreceiver Sending application ? snd_wnd next_to_be_sent sent & acknowledged not yet send sent but not acknowledged empty buffer TCP-buffer next_to_be_acked

6 Research TCP-RTM: semi-reliable protocol * Combines retransmission abilities of TCP and non- blocking behavior of RTP –Retransmission mechanisms reused from TCP –Packets that cannot be delivered in timely fashion are being skipped Application techniques resemble those for RTP (surveyed later) Requires minimum of TCP-code modifications, can be implemented as another TCP-option * Sam Liang, David Cheriton, TCP-RTM: Using TCP for Real Time Applications, Submitted to IEEE ICNP 02, 2002

7 Research TCP-RTM receiver – read-over-hole technique Receiving application ! Too late to resend - acknowledge step2 Receiving application step3 segment consumed by application segment received but not yet consumed by application skipped segment empty buffer Receiving application ? re-request on dup-acks step1 snd_cwnd

8 Research TCP-RTM sender Sending app NextByteToSend TCP SND_BUF Sending app NextByteToSend TCP SND_BUF Step 1Step 2

9 Research TCP-RTM – application techniques Receiver application doesnt read from the buffer until the data is needed for timely playback –this provides fixed play-back delay –timestamps are needed for this TCP-RTM supports application-level framing (ALF) –one application frame per TCP-segment – hence, always integral application frames are lost –every application write/read operation deals with 1 application level frame –ALF helps effective recovery from the losses

10 Research Wireless LAN – an example of a burst loss network Managed WLAN (NatLab), UDP packets, packet_size=500B, 100 packets/sec

11 Research TCP-RTM and burst losses empty buffer Receiving application ? step1 ?? snd_cwnd Receiving application (blocked) re-send on rto step2 ??? Receiving application step3 out-of-date segments consumed by application received out-of-date sergments sergments received but not yet consumed by application

12 Research Recovery price Burst loss size, packetsMinimum pbd to cover the losses, mS …-* buf_size = 64K, frame_size=1K, frames sent every period=20mS * - TCP-buffer gets full rto is doubled with every packet loss Near-exponential dependency – congestion window doesnt grow large enough to trigger the dup-ack retransmission

13 Research TCP-FCW vs TCP-RTM TCP-RTM wasnt meant to be used on burst losses networks –Losses of more than one segment in a row are not expected to happen –To be used on the real Internet –Be TCP friendly (use congestion avoidance) TCP-FCW meets different assumptions –To be used on the QoS enabled networks with bandwidth reservations –Send as fast as you can, but not faster than requested by the application Thus, we can control the bit-rate of the transmission on the application level –Provide immediate recovery – no slow start –Its not meant to be used on the current Internet

14 Research TCP-FCW: free-congestion-window Receiving application ? re-request – dup-acks again! step1 step2 step3 ?? ! resent packet consumed by application packet received but not consumed by application skipped packet empty buffer recovered packet

15 Research Evaluation Result: exponential growth of pbd in case of TCP-RTM vs linear growth in case of TCP-FCW

16 Research Further work TCP-MM (multi-media) – the content-aware transport protocol –Firstly - video –It will know about I,P,B frames of a video stream –Not segment, but video-frame-oriented

17 Research Questions